A scanning probe microscope is a microscope in which a probe needle detects physical or chemical actions from surface atoms of a sample while it is scanned across the surface so that the image of sample surface can be generated from the detected signals. Consequently, the resolving power and measurement accuracy of the scanning probe microscope depend to a large extent on the size and physical properties of the probe needle.
The use of a nanotube, typically a carbon nanotube (CNT), which is sturdy in ultra-minute diameter, for the needle probe of scanning probe microscope has made it possible to realize a high resolution. However, fastening of the nanotube on a holder to retain the needle probe has required very high level of microscopic processing technology.
As far as the nanotube probe and manufacturing method thereof are concerned, the first invention was that of Cobert Daniel Tea, Dye Hongy, et al as disclosed in Patent Publication 2000-516708. Subsequently, in the course of improving said invention, the inventor of the present invention disclosed Patent Publication 2000-227435.
In the patent publication 2000-516708, a nanotube is used as the probe needle for scanning probe microscope, wherein the nanotube is fastened to a protruded portion of a cantilever by means of an adhesive using an optical microscope. However, as the maximum magnification of an optical microscope is limited to 1000 to 2000 times, it was difficult, in principle, to observe directly a nanotube which is as small as 100 nm or less.
Thus, it was difficult even to fix the nanotube to a certain location on the protruded portion of a cantilever, and it was still harder to control the number of fixed nanotubes as well as the orientation thereof. To make the matter worse, it often happened that more than one nanotube were fastened, causing the obtained image to become overlapped or that the angle of the probe needle set with reference to the observed surface deviated far from 90° causing erroneous image to be generated. In other words, the situation was analogous to handling nanotubes in a dark room.
In an attempt to improve such a situation, the patent published 2000-227435 proposed assembly of a nanotube probe in an electron microscope while observing the nanotube directly. To be more specific, it provides a method for manufacturing nanotube probe accurately and simply by fastening the nanotube on the holder surface with coating film generated by irradiation of electron beam.
FIG. 15 shows a schematic diagram of an example of a nanotube probe constructed by the above conventional technology. While being observed directly under an electron microscope, a nanotube cartridge 106 on which nanotubes are adhered and protruded portion 104 of a cantilever are placed facing each other. Subsequently, the both members are caused to approach until the base end portion 108b of the nanotube comes in contact with said protruded portion 104. Here, the nanotube 108 has the tip end portion length A that is sufficient to be used as a probe needle while the based end portion 108b has a base end portion length B.
Next, when an electron beam 110 is irradiated, the impurities floating in the sample chamber of the electron microscope are decomposed, and a coating film 112 is formed with carbon substances, which are generated by re-composition of the decomposed substances. By this coating film 112, the base end portion 108b of the nanotube is fastened to said protruded portion 104 of the cantilever.
As illustrated in FIG. 15, the electron beam 110 has a beam diameter that covers entirely said base end portion 108b of the nanotube. Therefore, impurities 142, 142 existing in the sample chamber are decomposed by the electron beam 110, with the generated carbon substance forming the coating film 112 that covers the base end portion 108b of the nanotube.
However, said carbon substances do not only form the coating film 112, but they also are scattered because they are charged by electrons 140, 140 and repelled by each other. In addition, the debris of the carbon substances are also scattered. Thus, it often happens that impurities 136 stuck to other areas than the coating film 112, and thereby stain the protruded portion of the cantilever. Furthermore, in case that the coating is applied on the entire base end portion 108b of the nanotube, the end face diameter of the electron beam 110 is required to be very large, lowering the energy flow density, which necessitates irradiation of electron beam 110 for very long time.
Furthermore, when the tip end portion length A is established at an appropriate length, the base end portion length B can sometimes be considerably long as shown in (15B). If the base portion length B is larger than the beam diameter, in order to provide full coating of the base end portion 108b, it is necessary to use a multi-step formation of the coating film 112 by moving the electron beam 110 in the direction of the arrow m. However, the larger is the covering area of the coating film, the longer would be the time for fastening operation with according increase of impurity 136 that adheres to the protruded portion of the cantilever. As the result, some of said nanotube probes were too much stained with the impurities to be offered as a commodity.
FIG. 16 shows a schematic diagram of another defective nanotube probe as well as an illustration thereof in measuring operation. Once the nanotube 108 has been fastened to the protruded portion of the cantilever 104 in a state wherein the nanotube does not pass the sharp end of the protruded portion by way of one-shot coating over full length of the base end portion of the nanotube, it is impossible to correct such displacement of nanotube. Such displacement is liable to happen when the nanotubes have been arranged obliquely on the nanotube cartridge 106.
Such displaced fastening of nanotube can lead to a problem that the sharp end of the protruded portion 107 may function as a probe needle as well as the end of the nanotube 108C in an AFM measurement. If the nanotube 108 is displaced from the sharp end scanning point 150 and the nanotube scanning point 152 may pick up dual image, resulting in erroneous information of sample surface 148.
FIG. 17 shows a schematic diagram of a conventional cantilever having protruded portion 104 with curved side face as well as a nanotube 108. In case the side face 122 of the protruded part of the cantilever 104 is curved concavedly, it is required to fasten the nanotube 108 on the curved side face 122. With clearance between the protruded portion 104 and the nanotube 108, the overall coating film 122 can provide effective fastening only at the vicinity of the sharp tip end and the bottom of the protruded portion, leaving all other areas of coating film not contributing to fastening. As described above, there are many areas that need to be improved in the conventional overall coating method.
Accordingly, the object of the present invention is to provide a nanotube probe that is fastened to a holder with strength equal to or greater than a prescribed level and that allows fastening time to be shorter, and further to provide method for manufacturing thereof. Furthermore, another object of the present invention is to provide a nanotube probe that suffers minimum adherence of impurities in fastening process, and that can be fastened with increased strength, and further provide method for manufacturing thereof.